It is known to heat treat ferritic alloy steel to obtain properties. It is known also that the rate of cooling during heat treatment is the most critical aspect and it is through the rate of cooling that different properties will obtain. Heat treatment may comprise a single process or combination of processes.
In heat treating ferritic alloy steel, there are two transformation temperature ranges: one for heating and one for cooling. The heating transformation temperature range is denoted by the range Ac.sub.1 -Ac.sub.3, where Ac.sub.1 is the temperature at which austenite begins to form and Ac.sub.3 is the temperature at which the transformation of ferrite and cementite to austenite is complete. The cooling transformation temperature range is denoted by the range Ar.sub.1 -Ar.sub.3, where Ar.sub.1 is the temperature at which the transformation of austenite to ferrite plus cementite is complete and Ar.sub.3 is the temperature at which austenite begins to transform to ferrite plus cementite.
Two well known heat treatment methods are normalizing and tempering:
Standard normalizing is a heating and cooling process for refining the grain size of ferritic alloy steel. That is, the process makes the steel's microstructure more uniform. Standard normalizing is performed by heating the steel to a temperature above Ac.sub.3 and then air cooling such steel to room temperature. This process provides moderate hardening. The microstructure of as standard normalized AISI 4130 grade alloy steel is pearlite plus ferrite.
Tempering is a reheating and cooling process for softening ferritic alloy steel (viz., decreasing hardness, tensile strength, and yield strength), toughening the steel, and increasing the steel's ductility (viz., % elongation and % reduction in area). Tempering is performed by heating the standard normalized ferritic alloy steel to a temperature below Ac.sub.1 and cooling the steel to room temperature at any desired rate. Tempering causes no significant effect on the microstructure of AISI 4130 grade alloy steel.
The present invention provides a heat treatment process that imparts properties to ferritic alloy steel that are an improvement over those imparted by standard normalizing or tempering alone or in combination.